1. Field of the Invention
This invention relates to a method for coating the surface of fine particles closely with a layer of fine metal oxide powder several nanometers in thickness. More particularly, this invention relates to a method for coating the surface of fine particles by causing fine particles having water adsorbed in advance on the surface thereof to contact a metal alkoxide thereby hydrolyzing the metal alkoxide and consequently covering the particles with the hydrolysate.
The method of this invention has utility in such applications as production of a sintering auxiliary.
2. Description of the Prior Art
As methods heretofore commonly employed for the production of granules coated with a layer of fine metal oxide powder several nanometers in thickness, there can be mentioned a method which comprises mutual adsorption between particles of a microfine powder not more than approximately 10 nm in diameter and granules approximately 1 .mu.m in diameter through the medium of a surfactant by controlling the pH value and adjusting the zeta (.zeta.) potential [S. Hirano, Design of Complex Material II Organic Complex Material, (Progress of Chemical Industry 23 Molecule Chemical Industry Chapter 7) edited by Chemical Engineering Society], a method which comprises thoroughly mixing a microfine powder and granules in a liquid and spray drying the resultant mixture [Y. Kawashima, Journal of Color Material Society, Vol. 55(9), p 657 (1982)], and a method which comprises subjecting a microfine powder and granules to mutual friction in dry air thereby attaining mutual adhesion therebetween by virtue of static electric force [F. Yokoyama et al, Abstracts of Papers Read at Granule Formulation and Design Symposium p 67 (1986)]. In case of these conventional methods, the success of the deposition of a microfine powder on granules is at the mercy of probability and control of layer thickness requires considerable know-how. In the products of these methods, the coating layer is a coarse film formed by mere cohesion of the particles of microfine powder and the percentage of the microfine powder which escapes cohesion is large. The methods further have a disadvantage that they are not capable of easily producing a submicron order coat of high economic value.
In the circumstances, it is necessary to develop a method capable of coating fine particles with a dense layer of microfine powder without entailing any of the drawbacks mentioned above.
To this end, the present inventors earlier proposed a method for forming a dense layer of a metal oxide on fine particles by causing fine particles having water adsorbed in advance on their surface to react with a metal alkoxide incorporating a small amount of a higher fatty acid therein (U.S. application Ser. No. 07/496,060). With this method, however, there is the possibility that part of the metal oxide formed during the hydrolysis will peel off, depending on the conditions for the formation of the coated fine particles.